RFC 1195 [IS-IS for IP Networks] describes the use of default routes for each router in an IS-IS area to reach a border router between the level 1 IS-IS area of the router and the level 2 IS-IS domain. According to RFC 1195 a default route must be determined for each L1 router to reach the closest reachable L1/L2 border router in the network advertising attach (ATT) bit in its link state protocol (LSP)-0 fragment. All L1 routers in the IS-IS area would do this during their respective convergence process with the next hop of the default route set to the adjacent router through which the closest L1/L2 border router is reachable.
When the adjacent router or the link to that adjacent router designated as the next hop for the default route fails, traffic disruption will occur until the network re-converges. This disruption of traffic can be prevented by the Loop Free Alternatives (LFA) technology in principle as specified in RFC 5286. However, the specification (RFC 5286) doesn't specify any procedure that can be applied to the default route in particular. In many instances the standard LFA process however will not identify an optimal LFA for default routes.
The LFA that are identified with the standard process may fail to identify possible LFA that provide better coverage and better level of protection thereby causing increased traffic disruption in an IS-IS area in response to failures of links or nodes in the IS-IS area.